Understanding the effects of uv and ionizing radiation on cellular processes at the molecular level still remains a major task in Radiation Biochemistry. It is especially important to understand the molecular basis for the radiation sensitivity of dividing cells since rapidly dividing cell populations are particularly sensitive to irradiation. Since the polymerization of tubulin into microtubules is a process associated with the formation of the mitotic spindle and cell division, it is important to know the sensitivity of this process to irradiation. The experiments described in this application are designed to provide this information which at the present time is unknown. Toward this goal the following are planned: 1. Action spectra will be determined for inhibition of the polymerization, kinase, and acceptor activities of purified tubulin in vitro. 2. The relative sensitivity of the polymerization, kinase and acceptor activities of purified tubulin to different doses of X-irradiation will be examined. 3. The type of molecular damage leading to inhibition or inactivation of each of the activities of tubulin will be determined by examining irradiated material for changes in amino acid composition and conformation. The methods employed include amino acid analysis, absorption and fluorescence spectroscopy, fluorescent dye titrations, and disc gel electrophoresis.